1. Field of the Invention
The present invention relates to a polypropylene composition having an improved radiation resistance. More particularly, the present invention relates to a polypropylene composition which does not show substantial reduction of the physical properties or substantial discoloration even long after the composition is irradiated with radiations such as .gamma.-rays or electron beams, and which is suitable as a food packaging material or a medical instrument.
2. Description of the Prior Art
Polypropylene is widely used in various fields and is used especially effectively as a food packaging material and film and recently as a medical instrument such as a syringe. Generally, various stabilizers are incorporated into polypropylene so as to impart a good stability at the molding step and a good practical durability to a molded article. When a vessel or film having a food or farm product packaged therein or a medical instrument is used, an irradiation treatment with radiations such as .gamma.-rays or electron beams is generally carried out so as to attain a sterilizing or insecticidal effect on the vessel, film or medical instrument inclusive of the content or to prevent germination of the content. However, a polypropylene composition having a stabilizer incorporated therein is defective in that after irradiation with radiations, the mechanical strength is reduced or discoloration is caused. If this polypropylene is irradiated with .gamma.-rays at a dose of 2.5 Mrad which is ordinarily regarded as a safe and effective dose for sterilization of medical instruments, reduction of the mechanical strength, which is due to decomposition and deterioration of polypropylene, is conspicuous.
As means for solving the problem of the radiation resistance, there have been proposed a method in which a phenolic antioxidant having an isocyanurate group and a thio co-reagent are incorporated in polypropylene (Japanese Patent Application Laid-Open Specification No. 137135/80), a method in which a triaryl phosphite type antioxidant is incorporated in polypropylene (Japanese Patent Application Laid-Open Specification No. 179234/82), a method in which a heterocyclic hindered amine compound and 1,3,5-tris(3-hydroxy-2,6-dimethyl-4-alkylbenzyl)isocyanurate are incorporated into polypropylene (Japanese Patent Application Laid-Open Specification No. 42638/83) and a method in which a plasticizing additive such as liquid paraffin or a phthalic acid ester oil and a heterocyclic hindered amine are incorporated into polypropylene (Japanese Patent Application Laid-Open Specification No. 201849/83).
In the polypropylene compositions obtained according to these methods, the physical properties are improved to some extent over those of the conventional polypropylene compositions. However, when these improved polypropylene compositions are used in the above-mentioned application fields, they are still insufficient in the processability and physical properties.